Fluid Power Reference Guide - Hydraulics and Fluid Handling

April 3, 2003
This MACHINE DESIGN Reference Guide looks at recent innovations in fluid power, control, and handling.

Fluid Power Reference Guide

Hydraulics and Fluid Handling

Steering controls feature innovative connections

Two new families of steering-control units from Eaton Corp., Eden Prairie, Minn. (www.eaton.com), are said to provide responsive steering and crisp centering, plus flexibility and quick installation for OEMs.

The Char-Lynn Series 5 Steering Control is designed for low-flow, low-pressure applications such as lawn and turf equipment, lift trucks, and small and medium-size utility vehicles. Compact designs, a square housing with side ports and a round housing with side or end ports, provide several installation options. Various configurations include an integral-column option that eliminates the need for external columns. Series 5 is also said to offer better efficiency (lower pressure drop) than competitive units.

The Series 10 Steering Control is for moderate-flow, high-pressure applications. It has a 4,000-psi continuous pressure rating and a wide-walled sleeve 40% thicker than previous versions, which permits high-pressure operation. Options include a complete line of integral valves and bolt-on priority valves. Target markets are agriculture, construction, industrial, and material-handling equipment.

An option for both series is the Aeroquip Snap-To-Connect (STC) connector, designed to eliminate leaks, improve accessibility, and reduce installation time.

The threadless connectors are an alternative to conventional threaded fittings commonly used in fluid-power systems. The male and female halves simply push together -- without wrenches or special assembly tools -- to form a leak-free connection. A latch ring ensures positive engagement of the two ends.

In the Series 5 and 10 units, STC ports are machined into the housing. The "snap-in" connection cuts assembly time from several minutes to literally seconds and eliminates torquing inconsistencies that can cause conventional threaded fasteners to leak. STC connectors also rotate during assembly, eliminating twists that dramatically shorten hose life.

The fittings are said to weigh less and have a lower profile than threaded connectors. Working pressures are to 6,000 psi, with dash sizes ranging from -04 to -16.

STCs are used on trucks, buses, and agricultural, construction, material-handling, and turf-care equipment. Typical applications include hydraulically assisted fuel injection, power steering, hydraulic and air brakes, oil cooler, and high-pressure oil lines. Additional information can be found at www.aeroquip.com.

Photo: Char-Lynn Series 5 and 10 control units provide responsive steering in a variety of vehicles.

Fast valve offers high-flow

Accurately controlling high flow in hydraulic systems can be difficult. One reason is that high loads often affect valve performance. A new direct-drive, servoproportional valve from Moog Inc., East Aurora, N.Y. (www.moog.com), is said to offer more-precise control, lower hysteresis, and higher sensitivity at high flow rates compared to conventional valves with proportional solenoids.

High spool-driving forces, an optimized spool geometry to compensate for flow forces, and a bidirectional linear-force motor hold maximum flow nearly constant at high pressure drop. Yet dynamic performance is nearly independent of system pressure, so the valve is also suitable for low-pressure applications.

Another advantage of high spool forces is it provides better resistance to contamination. And in the event of power failure, the spool returns to its spring-centered position without permitting flow to an actuator. This prevents unintended machine movements and improves safety. The spring-centered design has no pilot oil flow, so it saves energy. And onboard electronics simplify setup. The D634-P valve is available in 24, 40, and 60-lpm (6, 10, and 15-gpm) versions at a pressure drop of 5 bar (75 psi) per land.

The D634-P is a high-speed, high-flow addition to the company's Direct Drive valve line, designed for precision-control applications in the wood-products, plastics, and metalforming industries.

Photo: The D634-P servoproportional valve offers precise control in high-flow applications.

Flow fuses safeguard hydraulics

Adjustable flow fuses, such as the Series AVF from Parker Hannifin Hydraulics Div., Elyria, Ohio (www.parker.com), are valves designed to shut off flow in hydraulic lines should tubing rupture or a hose fail.

Under normal conditions, a spring in the valve holds a poppet open and permits free flow in either direction. However, if differential pressure across the valve exceeds 45 psi, the poppet closes and quickly stops oil flow. Conditions that produce high-pressure differentials include broken lines, overspeed caused by feedback failures in closed-loop systems, and higher-than-normal flow. The valves adjust for a wide range of flow within a given port size and, once adjusted, tightening a simple set screw locks the setting.

The valves are ideal where, for safety or other reasons, a hydraulic line must close immediately. A typical application involves marine-industry hatch lifts. The valve mounts in the hydraulic line leading to an actuator and during normal operation, fluid flows freely in either direction. If the hydraulic line ruptures, however, the valve senses the change in differential pressure, shuts off flow, and prevents the hatch from falling. After repairing or replacing the line, the valve automatically resets to permit normal operations to resume.

Other installations include the primary pressure lines of central hydraulic systems, such as those on large drilling platforms. Should the line between the pump and control valves fail, the flow fuse shuts off flow. This limits oil spillage, environmental contamination, expensive cleanup, and the cost to replace lost oil and repair a damaged pump.

Information for this story provided by Don Caputo of the Parker Hannifin Hydraulics Div., Elyria, Ohio

Photo: Series AVF flow fuses are rated for a maximum working pressure of 5,000 psi. Designed for in-line installation, they are available in six standard pipe sizes ranging from 1/4 through 11/2 in. with a choice of NPT, SAE, or BSPP ports.

Cylinder protects mobile-hydraulic position sensor

Construction, mining, and other mobile equipment can be especially rough on sensitive equipment. A new position sensor that mounts inside a hydraulic cylinder is protected from the rigors of on and off-highway use. The MH magnetostrictive sensor from MTS Sensors, Cary, N.C. (www.mtssensors.com), is said to give OEMs and cylinder suppliers a viable option for noncontact position feedback in new-generation "smart" hydraulic actuators.

As with other MTS Temposonics position sensors, each MH sensor has three primary components: a magnetostrictive sensing element, electronics, and a housing that protects internal components. The compact housing comes in 7 and 10-mm diameters, with static pressure ratings of 4,300 and 5,000 psi, respectively.

Measuring range is 50 to 1,000 mm with an accuracy of 0.005%, suitable for most mobile-sensing applications. The sensor accepts 10 to 32-Vdc power and produces a direct analog-displacement output of 0 to 5 Vdc. According to MTS, the MH meets stringent shock, vibration and electromagnetic immunity specifications for on and off-highway vehicles.

Photo: The MH position sensor mounts inside a hydraulic cylinder, protected from the rigors of mobile applications.

Digital cylinder control

A new digital controller has been introduced for hydraulic cylinders on injection-molding machines. The DPQ Digital Controller from Bosch Rexroth, Hoffman Estates, Ill. (www.boschrexroth-us.com), uses advanced control algorithms that are said to improve system response, accuracy, and repeatability compared with previous controllers.

Users select internal or external (analog voltage) injection-profile control. The DPQ features closed-loop injection velocity and pressure profiles with smooth transitions that eliminate hydraulic shock. Injection-cylinder position or mold-cavity pressure can trigger the switch from velocity to pressure control. The controller's WinHost software includes data acquisition and real-time graphing. A PC interface is optional -- users can commission the DPQ via a faceplate display and push buttons.

The unit is flexible enough for use with most injection-molding machines. In addition, the DPQ can control cylinders for machining operations such as broaching and honing, and compression-molding presses.

Photo: The DPQ provides accurate and repeatable control of cylinder position.

Cylinders built for durability

The Cylinder Differential Tie Rod (CDT4) Series of NFPA hydraulic cylinders from the Bosch Rexroth Industrial Hydraulics Div., Bethlehem, Pa. (www.boschrexroth-us.com), are built for durability and long life.

Several features are said to enhance reliability. For instance, the cylinder has a removable, ductile-iron rod bearing that reduces wear and extends service life. Minimal guide clearance provides uniform load distribution, and a spiral groove in the rod bearing keeps surfaces lubricated and compensates for pressure changes.

A polyurethane seal system is standard, but other seals are available for HFC (water glycol), phosphate ester, and low-friction applications. An optional Exact-a-just cushioning system lets users accurately adjust end-of-stroke conditions for faster cycling, lower internal and external shock loads, and reduced maintenance costs.

The CDT4 Series features ISO 11926-1 SAE straight-thread ports and an air bleed designed to lock in place and resist coming loose. Piston-rod diameters range from 5/8 to 5 in. and bore diameters are from 11/2 to 8 in. Maximum pressure rating is 3,000 psi.

The cylinders are used in machine tools, presses, plastics machinery, material-handling and automotive equipment, as well as wood and paper manufacturing and civil-engineering applications.

Photo: CDT4 NFPA-style cylinders have a durable design suited for manufacturing, industrial, and civil-engineering applications.

Diaphragm pump features two-in-one valve

Oscillating-diaphragm pumps provide a compact, economical, chemically resistant way to transfer or recirculate fluid. The Oscillating Diaphragm pump from Gorman-Rupp Industries, Bellville, Ohio (www.gripumps.com), features a unique duckbill/umbrella valve that requires no external sealing and has no dynamic seals, reducing the potential for leaks and failure.

The heart of the pump is an armature assembly that extends from an electric coil and attaches to an elastomeric diaphragm. An electronic circuit feeds dc pulses to the coil 60 times/sec. Each pulse draws the armature and diaphragm toward the pump head, forcing liquid through the duckbill valve and out the discharge port. Springs then return the armature and diaphragm, pulling liquid through the umbrella valve from the pump inlet.

All wetted parts are constructed of FDA-approved materials, including the silicone valve and EPT/EPDM diaphragm. Flow rates range to 350 ml/min, discharge pressure to 35 psi, and the pump can self prime to 5 ft. Maximum continuous fluid temperature is 104°F, although higher intermittent temperatures are permitted. Standard motors are 115 and 230 Vac, 50/60 Hz. Options include dc motors for pulse-width-modulated signals. Rated pump life is 4,000 hr in continuous duty.

The units are suited for various applications, including food-products machinery, dishwashers, ice machines, chillers, and medical, analytical, and laboratory instruments.

Photo: Chemically resistant oscillating-diaphragm pumps are suited for food, medical, and analytical fluid-transfer applications. A key feature of the oscillating-diaphragm pump is a duckbill/umbrella valve. Fluid enters the pumping chamber through the umbrella valve and discharges out the duckbill. Building two valves into one elastomeric part results in fewer components, simpler assembly, and smaller package size.

Pulse-free metering pump

Pump manufacturers of all kinds -- from gear and reciprocating-piston to syringe and liquid-chromatography -- claim their products provide continuous, pulse-free fluid flow. In fact, almost none actually do, according to officials at Quizix Inc., North Highlands, Calif. (www.vindum.com).

The company's new QX pump uses two independently controlled, motor-driven pistons for applications requiring precise, continuous, pulse-free flow. The key is a prepressurization phase built into each piston stroke. While one cylinder delivers fluid, the other cylinder quickly refills and then strokes slightly. This prepressurizes the fluid to match the pressure in the first cylinder. Pressure transducers on the cylinders, a dedicated microprocessor, and sophisticated software algorithms ensure precise measurement and pressure matching. As a result, there is no detectable pulse at cylinder switchover.

Prepressurization also eliminates volume-measurement errors due to fluid compressibility. Because the fluid prepressurizes before delivery, the controller knows how much of the stroke actually delivers fluid. Conventional pumps cannot differentiate between fluid compression and delivery, thereby introducing volume-measurement errors.

The pump operates in several modes, including continuous flow at a constant rate or pressure. A ramping mode provides a smooth transition between two rate or pressure settings, and the pumps can also operate in either direction to deliver or withdraw fluid.

Applications that require continuous flow of several fluids can use two or more QX units. Those that deliver a small amount of two fluids can use a single pump and operate each piston independently. A computer interface using the company's PumpWorks software or a touchscreen front-panel display controls flow, pressure, and all other operating parameters. The software runs up to eight pumps simultaneously or independently.

The metering pump is available in three versions with maximum pressure ratings of 500, 1,500, and 6,000 psi. Maximum flow rates are 500, 200, and 50 ml/min, respectively. Minimum flow rate is 0.001 cc/min, and flow is said to be accurate to ±0.1% of the set rate, regardless of pressure, temperature, or fluid.

The QX comes in Hastelloy or stainless-steel models. An optional washout system cleans residual material off the piston assemblies to minimize corrosion and extend seal life. The pumps are suited for applications such as catalyst and chemical injection, pharmaceutical research, and core analysis.

Photo: The QX pump features two independently controlled pistons that continuously deliver a single fluid, or can pump two different liquids.

Chemical marker pinpoints oil's age

A major headache with industrial lubricants is monitoring oil condition. It can look like new yet contain harmful contaminants, or appear dirty while working fine. Fluids that are part of an integrated oil-analysis program from Dow Corning, Midland, Mich. (www.dowcorning.com), address this concern. They contain a proprietary chemical marker that pinpoints when to change plant lubricants.

The lubricant contains a sacrificial antioxidant additive that gives an indication of the oil's age. Tracking it lets users predict the condition of the remaining oil. The system also monitors other key variables to determine how lubricants have aged, unlike most oil-analysis programs that only test for wear metals or corrosion. These include:

Total acid number (ASTM D-974) levels that cause fluid oxidation, a key indicator of lubricant condition.

Particle count and water levels that indicate system cleanliness and filter performance.

Metals analysis, to detect component wear and monitor additives.

Kinematic viscosity, a key "aging" indicator that reflects lubricant condition. Viscosity tends to increase as lubricants age.

Computer analysis integrates the data with information about the lubricant and application. The output advises whether to replace the oil or continue operating with a recommended schedule for future sampling. This program is said to reduce the time and money wasted by changing oil too soon, or the potential for equipment damage by waiting too long. The result can be longer equipment life and lower overall maintenance costs.

The oil-analysis service is offered as part of the company's Molykote lubrication-management program. It includes a comprehensive line of industrial fluids, oil-analysis services, and lubrication-management software. Visit www.molykote.com for more information.

Modular designs streamline process analyzers

In the past two decades, analytical instruments have become more sophisticated and reliable, and the computers and software that operate them have become more powerful and user-friendly. However, the basic design of analyzer sampling systems has changed very little. These systems consist of valves, connectors, filters, seals, and sensors that work in concert to precisely deliver fluid samples to the analytical devices.

Unfortunately, they account for as much as 80% of the problems associated with process-analyzer systems. Thus, there is an incentive to improve sampler-system performance, and at the same time reduce design, manufacturing, and operating costs. A way to address all these needs is to make sampling systems smaller, modular, and more intelligent.

One organization at the forefront of this effort is The Center for Process Analytical Chemistry (CPAC), an industry-academic research consortium at the University of Washington, together with users and suppliers of process analytical-chemistry instrumentation. CPAC has announced the New Sampling/Sensor Initiative (NeSSI) with the goal of developing next-generation modular sampling systems.

A key element to NeSSI is open, standard architecture. For instance, The Instrumentation, Systems, and Automation Society (ISA) has developed a new elastomeric-seal standard, ANSI/ISA 76.00.02 2002, that applies to fluid-control components and fluid-path substrates. Different manufacturers are offering compatible approaches to substrate design, proving the feasibility and practicality of modular NeSSI Generation I systems.

Another advantage to the modular approach is faster and less-expensive development and layout, through tools such as system-configurator software. Modular systems are also easier to assemble and installation requires fewer connections. The designs tend to be smaller and lighter, so they can be closer to the sample point and reduce the need for long, heated, sample-transport lines. Smaller units also have less internal surface area than traditional systems, reducing the potential for fluid adsorption. And smaller internal volume makes it easier to purge and conserve expensive analyzer fluids.

Generation I systems that are in use confirm the intended improvements and cost savings. For example, new modular-platform components from Swagelok, Solon, Ohio (www.swagelok.com), streamline the development and assembly of complete, ANSI/ISA-compliant, process-analyzer and sample-handling systems. The approach cuts design, manufacturing, and maintenance costs and lets designers reduce size, weight, and flow-path volume.

The next phase, NeSSI Generation II systems, will address connectivity, communications, and intelligent-control issues. Key features will include compact and "smart" pressure, temperature, and flow sensors; valves with built-in electropneumatic actuators; multidrop sensor-bus communications, which may be wireless; and a sensor-actuator manager to enable system-wide control and communication.

Development of Generation II systems is underway. Testing will begin when basic communication issues are resolved and certain smart devices become available.

Information for this story provided by Dave Simko of the Swagelok Co., Solon, Ohio.

The Swagelok modular platform consists of manifolds, substrates, and surface-mount components such as filters and shut-off, needle, toggle, and check valves. Configurator software lets users design, place, and connect surface-mount components on a computerized layout grid. The software identifies the required flow connectors and generates a bill of materials and an assembly diagram.

Materials for extreme sealing

Standard seal materials usually cannot handle harsh or corrosive environments. A family of materials developed by the Parker Hannifin Seal Group, Cleveland (www.parker.com), is specifically designed for extreme-sealing applications.

Parofluor and Parofluor Ultra advanced perfluorinated elastomers (FFKM), formulated specifically for aggressive sealing applications, are said to outperform traditional fluoroelastomers and perfluoronated materials.

They feature temperature resistance to 608°F, high purity, compression-set and stress-relaxation resistance, and compatibility with a wide range of chemicals such as organic and inorganic acids, bases, amines, steam and hot water, ketones, and aldehydes.

The materials are used in O-rings, molded shapes, metal/elastomer composites, and other seal configurations. Various compounds offer a range of sealing performance levels. For instance, a low-closure-force version has a hardness of only 65 Durometer Shore A, so seals install easily into less-common groove shapes, such as dovetails and radius-corners. These soft seals compress with low closure forces, provide reliable sealing at low pressures, and are highly resilient with good compression-set resistance.

The Web site www.parofluor.com features technical information about Parofluor materials, as well as a collection of seal-engineering tools and resources. It includes data on chemical resistance, thermal stability, and semiconductor, aerospace, medical/biotech, and CPI/oilfield applications. A Design Tools section features an online fluid-compatibility guide, dimensional calculator, and utilities for failure analysis and pressure and temperature conversion. The site also lets users communicate directly with Parker's application engineers.

The company manufactures and packages the products in clean rooms to guard against contamination. The materials are suited for chemical processing, semiconductor fabrication, pharmaceutical processing, medical-device manufacturing, and other applications that require thermal stability, broad chemical resistance, and ultrahigh purity.

Photo: Parofluor materials are used in O-rings, as well as molded shapes and other seal configurations.

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